Scientists at the US Department of Energy’s Pacific Northwest National Laboratory have performed first-of-a-kind, high-resolution examinations of cracks in stainless steel core components from commercial nuclear reactors, dispelling many of the traditionally held beliefs about how cracks develop and spread.
According to the DOE, it has been long held that cracks in nuclear reactor components are the result of alloy embrittlement or local changes to the water environment caused by radiation. With the use of analytical electron microscopy and a new approach to access buried corrosion interfaces for study, however, Larry Thomas and Steve Bruemmer have discovered that the crack advance is promoted by radiation-enhanced, corrosion-induced material changes ahead of the tip of the crack.
While cracking of metal components inside nuclear reactors has been a continuing problem over many years, the ability to directly evaluate cracking mechanisms has been limited. Most research has been performed indirectly on non-irradiated components. With the new techniques developed at PNNL with support from DOE’s Division of Materials Sciences, Office of Basic Energy Sciences, these processes can now be directly investigated at the leading edge of crack propagation buried within the material.
Thomas and Bruemmer with the support of Clyde Chamberlin and other PNNL staff have developed the tools and techniques in laboratory radiation facilities to study the cracking phenomenon at the near-atomic scale.
Using high-resolution transmission electron microscopy they have discovered that the metal composition is altered over nanometer dimensions immediately ahead of the crack tip. Nanoscale observations of cross-sectioned crack tips in an irradiated stainless steel core component reveal unexpected morphologies and local chemistries never seen in non-irradiated steels.
This composition change is believed to promote crack advance and is caused by locally enhanced atomic movement due to excess space produced by the crack-tip corrosion process and neutron irradiation.
‘When materials crack, the leading edge of the crack is often buried deep inside the material and no one has ever been able to look inside at the point where the degradation occurs,’ said Bruemmer. ‘Now we can do it with great precision, allowing us to make a quantum leap in our understanding of corrosion and cracking mechanisms in a wide range of materials.’
A fundamental understanding of the cracking process is said to be an important first step in development of computer simulations to model propagation. It is also the first step in designing new alloys that can resist the corrosion process.
‘Most metals and alloys are susceptible to some degree of cracking in corrosive environments such as high-temperature water,’ said Bruemmer. ‘We may never be able to prevent cracks from occurring but these new insights could give us the tools to significantly delay the onset of degradation and reduce the probability of failure.’